99.98% of all relics are not what they claim to be.
Merely some relic hunter went on a mission to find St. JoeBloe's tongue or St Rottencrotch's britches and after 10 years of fruitless searching, simply cut off the piece from some random, plague-killed peasant or buy some bauble from a merchant that bears a passable resemblance (of course--no one ever knows what these are supposed to look like, anyway) and returns it to the contracted church for a handsome reward and a hero's welcome.
And this has actually been calculated, that if you gather all of the fragments of "the true cross" that can be found in reliqueries around the world, you would have, I think 17 "true crosses."